This invention relates to a power supply circuit of a switching inverter type and, more particularly, to a power supply circuit of this type capable of reducing switching loss to the maximum and improving conversion efficiency by utilizing both voltage resonance and current resonance as well as reducing noise by enabling operation waveforms of voltage and current appearing at respective component parts of the circuit to simulate a sine wave to a maximum degree.
Conventional power supply circuits are designed normally by using a large power transformer, connecting an AC power source to its primary winding and smoothing its secondary output by rectification. There is also a small, light and highly efficient conventional power supply circuit called a switching power supply source. In this type of power supply circuit, there is a switching inverter type power supply circuit of a half bridge type, a full bridge type or a push-pull type using switching elements which alternately turn on and off (e.g., transistors, thyristors or MOS-FETs). In the switching inverter type power supply circuit, operation voltage and operation current are both of a square waveform and this poses a problem of generation of noise. Besides, since this type of power circuit operates fundamentally in a high frequency, there arises a problem of heat generation due to loss caused by the fact that recovery time of a rectifying diode and off time of switching transistors are of a limited value.
In the switching inverter type power supply circuit, there has recently been made an attempt to reduce generation of higher harmonics, reduce noise and reduce also loss in circuit elements by causing either a current or voltage waveform to simulate a sine wave by utilizing resonance.
In the resonance type switching power supply circuit, however, on-off timing of switching elements, i.e., timing in which each switching element is turned on or turned off, the circuit must be accurately set and if setting of the timing is incomplete, improvement of efficiency cannot be hoped for. Therefore, it sometimes becomes necessary to detect a current in the circuit for determining the timing with resulting necessity for a complex circuit design. Moreover, in the resonance type switching power supply circuit, resonance is mostly used for causing either current or voltage to approximate a sine wave. Since noise in the resonance type circuit is generated by both voltage and current, the conventional resonance type power circuit is incomplete in the noise reduction.
It is, therefore, an object of the invention to provide a resonance type power supply circuit of a high efficiency and low noise with a simple circuit construction.